Clothing submerged in northern river? (9)
I believe the answer is:
underwear
'clothing' is the definition.
(underwear is a kind of clothing)
'submerged in northern river?' is the wordplay.
'submerged in' becomes 'under'.
'northern river?' becomes 'wear' (I've seen this before).
'under'+'wear'='UNDERWEAR'
(Other definitions for underwear that I've seen before include "what lies beneath that uniform!" , "Clothing nearest the skin" , "Smalls,shorts,lingerie etc" , "Combinations, say" , "Smalls or scanties, collectively" .)